The issue is that this._groupChatNode in GroupChatBookmark.as is null during serialize. Backtrace is
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at org.jivesoftware.xiff.bookmark.packet::GroupChatBookmark/serialize()[GroupChatBookmark.as:65]
at org.jivesoftware.xiff.bookmark.packet::BookmarkPrivatePayload/serialize()[BookmarkPrivatePayload.as:59]
at org.jivesoftware.xiff.privatedata.packet::PrivateDataExtension/_serializePayload()[PrivateDataExtension.as:59]
at org.jivesoftware.xiff.privatedata.packet::PrivateDataExtension/serialize()[PrivateDataExtension.as:51]
at org.jivesoftware.xiff.data::XMPPStanza/serialize()[XMPPStanza.as:116]
at org.jivesoftware.xiff.data::IQ/serialize()[IQ.as:75]
at org.jivesoftware.xiff.core::XMPPConnection/send()[XMPPConnection.as:305]
at org.jivesoftware.xiff.privatedata::PrivateDataManager/setPrivateData()[PrivateDataManager.as:42]
at org.jivesoftware.xiff.bookmark::BookmarkManager/_addBookmark()[BookmarkManager.as:124]
at org.jivesoftware.xiff.bookmark::BookmarkManager/addGroupChatBookmark()[BookmarkManager.as:47]
at com.jivesoftware.spark.renderers::MUCRoomListItemRenderer/addBookmark()[MUCRoomListItemRenderer.mxml:64]
I would start work from 'MUCRoomListItemRenderer.'